1. Field of the Invention
The present invention relates to a satellite communication (SATCOM) system and more particularly to a bi-static SATCOM relay for use with relatively low power earth stations, in which the receiver is carried by a separate hidden platform and interconnected with a transmitter platform by way of a secure narrow beam communications link.
2. Description of the Prior Art
Satellite communication (SATCOM) systems are generally known in the art. Examples of such SATCOM systems are disclosed in U.S. Pat. Nos. 5,433,726; 5,551,624 and 5,619,209, all assigned to the same assignee as the present invention and herein incorporated by reference. Such SATCOM systems are used in various commercial and military applications. For example, U.S. Pat. Nos. 5,433,726 and 5,551,624 relate to SATCOM systems for use with cellular communication systems, while U.S. Pat. No. 5,619,209 relates to a SATCOM system for use with a paging system. In such applications, a constellation of satellites in an earth orbit are used to provide communication links between various earth stations.
Other commercial applications of SATCOM systems are known. For example, “sky phones,” available on many commercial aircraft, allow passengers to communicate with terrestrial and cellular telephone systems. In this application, a satellite relay which includes a transmitter and receiver is used to form a communication link between the aircraft during flight and the terrestrial telephone systems.
Such SATCOM systems are also used in military applications. In such applications, satellites are used as relays to provide a communication link between various earth stations. As described in more detail in U.S. Pat. No. 5,528,247, hereby incorporated by reference, such SATCOM systems include a satellite relay 20 (FIG. 1) in an earth orbit which includes a transmitter and a receiver. The SATCOM relay 20 is used to provide a communication link between various earth stations 22 and 24, which may include mobile ground and naval terminals. In such applications, high powered jammers 26 are known to operate within the theater. In such a situation, the jammer 26 has several advantages compared to the earth stations and particularly mobile terminals. For example, the jammer 26 may have access to a relatively powerful generator which easily supports multi-kilowatt class transmitter amplifiers, such as traveling wave tube amplifiers (TWTAs), which are readily available in most uplink bands. In contrast, a mobile terminal is often battery operated which limits the amplifier power to about 10 watts or less. In addition, the jammer 26 can utilize a relatively large parabolic reflector or phased array antenna while the mobile user is normally limited to a much smaller reflector or an omni-directional antenna. As such, the jammers' effective isotropic radiated power (EIRP) exceeds the users EIRP by a substantial margin, such as 70 dB or more.
The jammers are stationary while the mobile terminals need to communicate on the move. As such, a high gain narrow beam jammer antenna can be set up and steered to the satellite receiver with great precision while the mobile terminal is forced to either trade away antenna peak gain for broader beam width in order to offset uncompensated dynamic pointing error or implement an antenna control system capable of tracking its motion.
Various techniques are known to have been tried to overcome the disadvantages of mobile terminals in such applications, such as spread spectrum processing gain and spacecraft receive antenna spatial discrimination. Unfortunately, both techniques provide relatively limited results. For example, it may be desirable for single mobile users with data rates up to 50 Mbps to operate in X-band with Defense Satellite Communication System (DSCS) waveforms. As such, less than 10 would fit within the 500 MHz SHF band allocation. With frequency hopping for anti-jamming protection, the maximum processing gain available to any user would be limited to about 10 dB. Even though more spectrally efficient waveforms provide higher gain at the expense of terminal complexity or higher terminal power, increasing the terminal EIRP also increases the user's vulnerability to detection and location by an adversary.
Alternatively, while spatial discrimination on the order 40 dB can be supported with current spacecraft antenna technology, the stand off distance, set by the SATCOM relay antenna at geosynchronous altitude, will not allow the earth terminal to be closer than a few hundred miles from the jammer 26, even when the user is near the sub-satellite point on the earth. The level of anti-jamming protection provided by spacecraft antenna beam shaping diminishes as the standoff distance decreases and as the user moves closer to the edge of the satellite's footprint. Antenna isolation becomes more challenging to maintain as the number of jammers increase within the antenna field of view.